In a wireless network a radio receiver is a function in a radio node for receiving radio transmissions from one or multiple radio transmitters. One of the most important and most fundamental problems is to set the output power of the transmitters. In many systems, e.g. LTE (Long Term Evolution), the radio links have a DL (downlink) and an UL (uplink) where a DL is a transmission from a “master” node, e.g. base-station node, that also schedules the radio link and a UL is a transmission from a slave node, e.g. UE (user entity), scheduled from the master node.
Typically, a base-station node sets the output power of the transmitters aiming for a determined received power in both UL and DL. The DL power often is a fixed power density at the base-station side according to the maximum power of the base-station. The UL power is determined by a procedure by which the base station transmits a DL reference signal that the UEs measure on and adapt their output power in order to make the received power density at the base station of all UL transmissions to be more or less the same. In LTE, how close the received power densities of different UL transmissions are, is determined by α in formula (1) below, where α=1 gives the same received power density. For the uplink data channel PUSCH (Physical Uplink Shared Channel) in LTE, the transmitted power by the UE in subframe i is determined by the formula:PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}  (1)where PCMAX is the configured maximum UE transmit power, MPUSCH is the number of resource blocks allocated for the UE, PO_PUSCH (j) is a parameter consisting of the sum of a cell-specific and a UE-specific part provided by higher layer, j can be 0 or 1, a is cell-specific parameter configured by higher layers (also known as fractional pathloss compensation factor), PL is the downlink pathloss estimate calculated in the UE, ΔTF (i) is a UE-specific parameter provided by higher layers and f(i) is UE-specific correction term controlled by TPC (Transmit Power Control) commands sent in uplink grants sent on the PDCCH (Physical Downlink Control Channel). For later releases of the 3GPP specifications power control is slightly more complicated due to the support of multi-carrier where the UE can support multiple serving cells.
The trend in advanced multi-antenna transmitters is that the common power amplifier (PA) that typically feeds all antennas is being replaced by distributed PAs, one per antenna element or subarray. In emerging systems, operating at higher than current cellular frequencies, e.g. mm-Wave, distributed PAs will be used not only for the base stations but also for the UEs, as it is likely that the radio will be integrated with the antennas. Power control for distributed PAs is then an even more complicated issue, due to the interplay with beamforming.
In current solutions for uplink power control with beamforming (jointly called precoding) in the case of distributed PAs, there are two dominant approaches, each of which has its advantages and disadvantages. The first approach is to use a phase-only precoder which is able to transmit at full power, i.e. exploiting equally well all PAs. From the perspective of a single link, this is needed in coverage limited situations. The problem with phase only precoding is that it has higher side-lobes (compared to an optimal precoder including amplitude tapering) and thus spreads more interference which has the effect of deteriorating the system performance.
The second approach is to find the optimum beamforming as if there was a common PA, but then normalize the output power in order not to saturate the PA with highest load. The effect is that the received signal quality deteriorates pre-maturely for cell edge users, either due to too low received power or also in some cases due to clipping. This is especially bad in code-multiplex systems, e.g. for uplink sounding signals in 5G for massive MIMO (Multiple Input Multiple Output).
Accordingly a need exists to provide an enhanced power control mechanism by which a defined signal quality can be maintained longer. Furthermore, interference with other communication entities shall be minimised.